This invention relates to alkali metal energy conversion devices, such as for example alkali metal cells and particularly sodium sulphur cells. Such cells typically employ a solid electrolyte element separating cathodic and anodic reactants which are liquid at the cell operating temperature.
A known construction of device comprises an external casing, a solid electrolyte element dividing the interior of the casing in to two electrode regions, an electrically insulating element joined to the electrolyte element, and at least one metal member sealed to the insulating element. This structure typically forms part of the sealing arrangement for the device, sealing off the two electrode regions both from each other and from the ambient environment. For example, the external casing of the device may be of metal, so that any sealing of an electrode region requires a seal to be made between the metal of the casing and the electrolyte element. However the metal of the casing must be electrically insulated from the electrolyte element and the insulation is provided by the intervening electrically insulating element.
An example of such an arrangement applied to a sodium sulphur cell is shown in GB-A-2102622 which has an alpha alumina lid closing a tubular electrolyte element. A centrally located current collector is mounted in an aperture through the alpha alumina lid and insulated by the lid from the electrolyte element. The outer electrode region, on the outside of the electrolyte element, is sealed by means of a metal closure member welded about its periphery to a metal casing for the cell, and sealed about an inner periphery to the alpha alumina lid. This latter seal between the metal closure member and the alpha alumina lid has hitherto sometimes been made by compression bonding using an intermediate layer, between the metal closure element and the ceramic lid, of a soft material, e.g. aluminium, to provide the necessary bonding.
It will be appreciated that the sealing of sodium sulphur cells and other alkali metal energy conversion devices is of critical importance in the manufacture of the cell to ensure good performance and safety and is a particularly difficult problem because of the high operating temperatures of these cells, typically 350.degree. C. Bonding techniques using cements have not proved practical.
The bonding techniques used hitherto to secure the metal closure element or elements to the insulating element have of necessity been performed after the insulating element is itself joined to the solid electrolyte element. The insulating element is typically a ceramic such as alpha alumina and the solid electrolyte is typically beta alumina. These are joined by glazing at elevated temperatures which would destroy seals between the metal element or elements and the insulating element made by prior art techniques.
This presents certain handling difficulties since the electrolyte element maybe relatively delicate and therefore great care must be exercised when performing compression bonding to the insulating element joined to the electrolyte element. Furthermore, the shape of the composite insulating element and electrolyte element may make it necessary to perform the bonding of the metal elements to the insulating elements individually on a cell by cell basis.